The invention relates to improvements in machines for processing plastic materials, and more particularly to improvements in injection molding machines. Still more particularly, the invention relates to improvements in machines for making tubular workpieces (such as cylindrical barrels of syringes) from plastic material which is injected into a cavity defined by a mold body and a core.
It is known to make the barrels of disposable syringes from a plastic material which is injected into the cavity between a core and a mold body in such a way that plasticized material enters the cavity in the region for the formation of the front end wall of the barrel, namely in the region of the end wall which carries the tubular extension or tip for the rear end of the hollow needle. The machine which can be used for such purposes can include a mold of the type known as cold runner mold or partly hot runner mold. A drawback of such proposal is that the barrel which is formed as a result of setting of plasticized material in the cavity invariably contains a sprue (waste portion). The sprue must be separated from the barrel and processed which normally involves classification and grinding for reuse of the plastic material. The mold must be designed to permit opening in two planes, namely in a first plane for the purpose of separating the sprue and in a second plane for the purpose of evacuating the barrel from the cavity. This necessitates the provision of a bulky mold.
In accordance with another prior proposal for the making of disposable syringes, plasticized material which is to form the barrel of the syringe is injected into the cavity of a hot runner mold in the region of the rear end of the barrel, namely at that end of the barrel from which the plunger or the piston rod extends when the syringe is fully assembled. This reduces the likelihood of the formation of sprue; however, air which must leave the cavity in response to injection of plasticized material can be expelled only along the narrow path which includes the hole in the tip or extension for the hollow needle of the syringe, i.e., by way of the central opening in the front end wall of the barrel. Abrupt expulsion of air from the cavity along such narrow path results in excessive heating of the customary pin which is installed in the mold to define the hole in the extension of the front end wall of the barrel. This often entails charring of the adjacent portion of the barrel so that the barrel must be discarded. In addition, it is necessary to inject plasticized material at a very high pressure in order to ensure that the expulsion of air from the cavity of the mold will be completed within a short interval of time. This contributes to the energy requirements and overall cost of the injection molding machine.
Another drawback of the just described molds for the making of barrels for syringes and like tubular objects is that the orifices which admit plasticized material into the cavity must extend all the way to the rear end of the cavity, i.e., to the end which is remote from the front end wall of the barrel. Since the plasticized material in the orifices is heated, the heating means must extend along the entire cavity so that the space requirements of such heating means are excessive. Therefore, the separable sections of the machine can define only a rather small number of cavities for simultaneous making of several barrels because the mutual spacing of neighboring cavities cannot be reduced at will in view of the need for heating means around each cavity and the respective orifices for admission of plasticized material. Still further, the heating means cannot be placed at a uniform distance from all portions of each cavity so that the material in the cavities undergoes a non-uniform heating action. This, in turn, entails undue deformation of the plastic objects because after shrinkage of material in a cavity is more pronounced in regions where the confined material was heated to a higher temperature.
As a rule, the means for heating plasticized material in the cavities of the aforedescribed hot runner molds is designed in such a way that several (e.g., four) closely adjacent molds are heated by a common heating unit which can include one or more heating rods. This exhibits the drawback that, if a heating unit is out of commission, all of the molds which are supposed to be heated by the defective unit are out of commission until after the defective unit is repaired or replaced. Such mode of heating groups of molds by common heating units can result in huge losses in output, for example, if a heating unit is damaged immediately after the start of a shift and the repair work cannot be carried out until after the shift is completed.
U.S. Pat. No. 4,184,836 to Rees discloses a multiple mold for producing elongated tubular articles. The mold body has several recesses for reception of discrete cores to define individual mold cavities. The arrangement is such that four cavities receive plasticized material from a common nozzle which is disposed between the recesses of the respective group. In order to ensure rapid distribution of plasticized material in the four cavities, the end walls of the tubular articles which are formed in the cavities are non-symmetrical, i.e., the extensions or tips for the rear end portions of the needles resemble inclined cones. If the articles are barrels of syringes, syringes embodying such barrels exhibit the drawback that their contents cannot be fully expelled in response to depression of the respective plungers. Moreover, admission of plasticized material from one side of the core in each recess entails a lateral shifting of the cores in response to the pressure of injected plasticized material, i.e., the thickness of the cylindrical walls of the barrels is not uniform which renders the barrels defective or, at the very least, unacceptable for certain applications.
British Pat. No. 909,555 to Rosedale discloses a method of producing thermoplastic tubes in a cold runner mold. Plasticized material is injected at the closed end of the cavity in the region where the material is to form the extension for reception of the needle. This mold exhibits the aforediscussed drawbacks, particularly the need for opening the mold in several planes in order to separate the sprue and to permit ejection of articles from their cavities.
U.S. Pat. No. 2,878,517 to Cramer discloses a mold for annular plastic articles wherein plasticized material must be injected along the entire inner portion of the cavity and the film-like sprue must be separated upon expulsion of the annular article from the cavity. This entails extensive secondary treatment of articles and involves removal of relatively large quantities of waste material, the same as in all machines which employ cold runner molds.
German-language article by Jorg Gaiser entitled "Spritzgusswerkzeug zu Herstellung dunnwandiger Hulsen" (Injection molding tool for the making of thin-walled sleeves) published in "Der Plastverarbeiter" (1974, Volume 25, No. 80, pages 473-475) proposes to mechanically hold the core during injection of plasticized material so as to ensure that the wall thickness of tubular articles will be uniform. The author proposes to inject plasticized material by way of two orifices and to form a so-called tunnel sprue which can be removed only if the mold is separable in a plurality of planes. The separated sprue must be ground for renewed use.